The reduction of CO2 emissions from fossil fuel power plants is an important effort for protection of the environment on the global scale. Coal based integrated gasification combined cycle plant (IGCC) technology enables production of electricity with a gas turbine utilizing a fuel that is rich in hydrogen and has a very limited amount of CO2. Combustion of a hydrogen-containing fuel requires dilution with a gas that in most cases contains nitrogen. As a result, a byproduct of the hydrogen-containing fuel combustion is a significant amount of NOx that can be reduced by using selective catalytic reduction systems in addition to low NOx combustors in the gas turbine.
Since fuel produced and used at an IGCC plant contains hydrogen (H2), it can also provide hydrogen for a reducing agent in the SCR process by introducing a small amount of H2 from the fuel supply into the SCR. The use of hydrogen as a NOx reducing agent allows elimination of ammonia as a reducing agent in the SCR system, and thus prevents discharge of ammonia slip into the ambient air, which is an inherent problem with current ammonia SCR technology.
It is known by those skilled in the art that H2-SCR is an efficient technology in O2-lean conditions, especially when amounts of water and sulfur compounds are limited to less than 5 vol. % and to less than 5 ppm; respectively. Reduction of NOx using H2 has the potential to generate reaction products that include both N2 and N2O. Obviously, catalysts with high selectivities towards the formation of N2 are preferred. It is known to those skilled in the art that the selectivity of Pt-based H2-SCR catalysts toward N2 formation is relatively low, and undesirable byproducts such as N2O and NH3 are produced.
Recently, a strong attempt to improve H2-SCR efficiency with respect to NOx removal and N2 selectivity under oxidizing conditions was made (U.S. Pat. No. 7,105,137). A developed Pt-based catalyst was durable for 24 hours when operating in a reaction mixture that contained 5 vol. % O2, 5 vol. % H2O, and up to 25 ppm of SO2. M. Machida et al., Applied Catalysis B. Environmental 35 (2001) 107, demonstrated that a Pt-based H2-SCR can have high selectivity to N2 under oxidizing conditions (10 vol. % O2) without H2O and SO2 present in the process stream. However demonstrations of the H2-SCR that ability to efficiently reduce NOx emissions were done for mixtures of gases have relatively low concentrations of O2, H2O, and SO2 or high concentrations of only one of O2, H2O, or SO2 which is contrasted to a gas turbine exhaust mixture from combustion of H2-containing fuels at IGCC plants.
Commercial processes such as Selexol™ can remove more than 97% of the sulfur from syngas. Still, the concentration of sulfur compounds in syngas can be up to 20 ppm. Taking into consideration dilution of syngas with nitrogen, the concentration of SO2 in IGCC gas turbine exhaust can be at the level of 5 to 10 ppm. After CO2 sequestration and burning of H2-fuel, concentrations of H2O in the exhaust can be as high as 20% by volume, and oxygen content can reach 6-10 vol. %. Under these conditions, developing a process to reduce NOx emissions in the exhaust of IGCC gas turbines by using H2-SCR is challenging. Thus, despite the above-described enhancements, there is a need to develop a process to reduce NOx emissions in gas turbine exhaust utilizing an H2-SCR that provides high NOx reduction efficiency at the level of 90+% with high (greater than 90%) selectivity to N2. Additional process requirements include extended durability and stability in presence of 10-25 vol. % of water, 5-10 vol. % of O2, and 5-10 ppm of SO2.